It has long been known that the length of thermoplastic fibers such as nylon and polyester change length during curing and related manufacturing processes. When such fibers are incorporated within pneumatic tires as reinforcing or stabilizing cords, successful manufacture requires an accurate allowance for such length change.
In order to quantitatively determine the magnitude of such length change, a single-cord, simulated cure/postcure inflation (SC/PCI) test device and method was devised which measured the effect on cord length of successive changes in temperature and load. The SC/PCI device is extremely simple in construction; a vertically-oriented cylindrical oven encloses a portion of a similarly oriented length of cord. The top end of the cord is fixed above the cylindrical oven, while a weighted pointer assembly pointing to a linear scale is applied to the bottom of the cord below the oven. A detailed explanation of the SC/PCI test may be found in the article by C.Z. Draves and L. Skolnik, Tire Cord Process Simulation and Evaluation, Kautschuk Und Gummi Kunststoffe, Vol. 22, No. 10, pp. 561-565 (1969).
Another method and device utilized to measure cord shrinkage is described in U.K. Patent No. 1,299,171, which is incorporated herein by reference. That manually operated device, made by Testrite Limited of Halifax England and commonly referred to as the Testrite cord shrinkage tester, orients the cord horizontally and, like the SC/PCI test, envisions heating only a portion of the cord to be tested. One end of the cord is fixed while the other end is wrapped around a drum carrying a pointer in front of a partially circular dial having a non-linear scale thereon. An interference fit between the cord and drum causes the drum and pointer to rotate with changes in cord length. The Testrite cord shrinkage tester by design is incapable of applying a significant tensile stress to the test cord and therefore is unable to perform shrinkage measurements requiring mechanical stress such as the SC/PCI.
The SC/PCI and Testrite cord shrinkage tests and implementing devices possess several deleterious features. The extensive processing time (about forty minutes) of the SC/PCI test limits the number of cords that may be tested during manufacture. Also significant are inaccuracies introduced by the devices themselves. One source of inaccuracy is the failure to maintain the cord under test at a uniform temperature which arises from placing only a portion of the cord under test within the oven. In the instance of the Testrite cord shrinkage test additional inaccuracies occur because of the non-linear scale used by the meter. Moreover, the manual operation of both testing devices produces results dependent on individual operator proficiency and requires considerable operator attention.